The belt conveyor is the single most important bulk material handling product and one of the oldest means for continuous moving of bulk materials. Initially it was designed and developed for in-plant use, primarily because of its ability to handle materials on a continuous basis. This eliminated older batch handling methods that not only used excessive power but were limited in capacity.
Over the years refinements in its hardware have been made, such as the application of anti-friction bearings in the conveyor support idlers, refinements in the sealing protection for the bearings, refinements in the construction of the belt itself and many others, all tending to broaden the application of the conveyor and make it an extremely reliable means of transportation.
The evolution of belt conveyors for high tonnage rates over long distances can be traced from the use of narrow belts at low speed, to wider belts, to still wider belts with larger load carrying cross sections, to higher speed belts, to very long steel cable belts capable of running at high speeds carrying large cross sectional loads.
Developments in machinery and belting moved forward together, from narrow 24 to 30 inch belts on idlers with concentrator roll angles of 20.degree. running at speeds of 300 to 400 F.P.M. over a distance of a few hundred feet, to 72 to 96 inch belts with idlers of 45.degree. concentrator roll angles operating at speeds as high as 1200 F.P.M. over distances in excess of ten miles in a single flight.
Just as the belt conveyor replaced in-plant batch handling, it is now replacing on-site batch handling at large earth or mineral moving operations and the transportation requirements are enormous. The on-site moving of millions of tons of earth to reach the desired commodity must be undertaken. Due to lesser concentration, more mined earth must be handled to obtain an equivalent amount of valuable ore. The distance necessary to move the ore to major transportation centers is greater because of the remote location of the deposit.
Basic design concepts, established during the development of in-plant use, have been refined for present day applications. Belt conveyors are now moving materials on a continuous basis, first, to meet the demands of increased size and overall operating efficiency of high capacity continuous excavators, and, second, to replace expensive inefficient mobile equipment such as trucks. Conveyors not only have the capacity to handle these large tonnage rates, but can do so at a lower cost per ton mile than other batch handling equipment. Broadening the scope of belt conveyor applications has resulted from increased operating speeds, increased sectional loading on the belt, and improved tension members of the belt. All three of these factors result in improved operating efficiencies, reduced initial costs and reduced operating costs.
Belt speeds have steadily increased over the years. Not too long ago speeds of 400 to 600 F.P.M. were common. Today it is not uncommon to see belts running up to 1200 F.P.M. Experimentally belts have been run as high as 2000 F.P.M.
In coping with speed limitations, conveyor designers have directed their attention primarily to problems at the terminals and not to the carrying run of the belt. Loading and discharge at high speeds must be given special attention to reduce chute wear, prevent plug-ups, direct the load to the center of the belt, provide for material acceleration to belt speed, minimize spillage, and minimize belt cover wear. As experience and skill solved one problem after another at the transfer points, bolder and bolder attempts toward higher and higher speeds were made. The main body of the conveyor, the section between the terminals, has been thought of as generally problemless as long as the terminals could be successfully negotiated. Recent experiments at high speeds have disproven this and may have determined that action of the material on the belt as it passes over the belt carrying idlers will determine the practical maximum speeds of belt conveyors for current belt conveyor design concepts.
Ever since the inception of the belt conveyor, the supporting idlers have been a series of rollers placed at right angles to the belt. These supporting idlers are spaced along the run of the belt at intervals of approximately four to five feet, or greater if the belt is under high tension. Obviously, a loaded belt has a tendency to sag between the spaced idlers, and it is this sag that dictates the maximum spacing of the idlers under the belt. A loaded belt moving across these spaced idlers at high speeds creates a wave in the material as it passes over each idler along its entire length. Under these conditions, a load in the belt is given vertical acceleration as it approaches each supporting idler to the extent that the load actually leaves the belt and repositions itself on the belt some distance upstream in the direction of travel. This throwing and catching action affects the material at each idler location. While the fact that the material leaves the belt and repositions itself is of no major consequence except that it does consume power, it is the phenomenon that takes place when the material repositions itself that is of concern. In repositioning, the material tends to build up a wave. Ultimately, on long conveyors the wave becomes so pronounced that at its crest the cross sectional load on the belt becomes excessive and spillage occurs, while a bare spot is created in the trough of the wave. This spillage, of course, would be intolerable. Such a condition begins to take place at belt speeds of 1800 to 2000 F.P.M. with conventional idler spacing.
The volumetric load carrying capacity for a given belt is the result of the contoured cross section of the trough and the speed of the belt, assuming the speed to be within the wave producing speed limit. The cross sectional load is determined by the cross sectioned contour formed at the troughing idlers.
For many years conveyor belts were made of cotton duck as the main tension carrying member with the weight of the duck and the number of plies determining the tension capability. Naturally, as the plies of the belt built up, the lateral stiffness increased. This lateral stiffness created a restriction on the maximum angle of the concentrator roll of the idler, thereby restricting the cross sectional load carrying ability of the belt. For many years, the troughing angle of a conveyor idler was limited to 20.degree. for "high capacity conveyors." Today, as a result of the development of manmade fibers and steel cord construction, lateral flexibility is maintained practically independent of the tension member requirement, which allows for greater troughing angles on the idlers even up to 45.degree.. Many installations are now in operation which do use the 45.degree. concentrator roll angles, but a deteriorating condition on the belt exists in the longitudinal flexing area where the idler center roll meets the concentrator roll.
When a troughed belt is loaded with material, a catenary sag is created between the idler supports and, as a result, at each idler the belt is caused to conform to an unnatural compound bend. The relatively stiff cross section formed by the deeply troughed thin belt, coupled with the catenary sag between idlers, is unable to yield to this unnatural condition. The center section of the belt in contact with the center roll flattens out and rides on the full width of the center roll. Where this flattened section joins the lateral troughed sides, a highly stressed deteriorating buckle is created. This condition is generated in the belt at each idler location, resulting in many stress cycles as the belt moves down the conveyor. Thus, while it is possible to use 45.degree. troughing idlers, prudent judgment suggests 30.degree. or 35.degree. idlers to extend belt life, even though there would be some sacrifice in conveyor capacity.
The theoretical maximum cross sectional load, the limit we should identify, requires the belt to be troughed in a continuous arc as shown in FIG. 1. This configuration cannot be achieved by the present conventional three roll idler. The closest approximation would be multiple rolls as exemplified in U.S. Pat. Nos. 3,880,275; 3,757,930 and 2,833,395, with economics dictating the actual practical number.
As previously stated, the evolution of conveyor belting has been the improvement of the tension carrying members while maintaining lateral flexibility. The allowable working tension of the belt is the factor which controls the maximum length of a single conveyor. Obviously, on long overland belt systems the longer the individual conveyors the fewer the terminals. Terminals are not only costly in initial investment; they are costly items of maintenance.
As a result of the development of belt tension members from cotton duck to high strength steel cord, belt lengths have increased from a few thousand feet to as long as ten miles. Even with steel cords, there will be some limit on the maximum length of a single flight conveyor. As yet, this may not have been determined, but it must be recognized that there will be some physical and economical limits.
The straight line conveyor run limitation is a factor that must be recognized. Training of a belt to run on the centerline of carrying idlers is important, as load spillage results if the belt wanders off to the side. In addition, the edges of the belt are likely to encounter fixed obstacles along the path or at head or tail terminals, resulting in tearing and fraying and ultimately the deterioration of the entire belt. Accurate alignment of idlers is therefore extremely important, especially when a belt is carried by fixed rigid frame support idler rolls.
As indicated, the speed limitation is caused by the "hump" that is created in a belt at each idler support, causing the material on the belt to reposition itself. The greater the spacing the larger the hump and, conversely, the smaller the spacing the smaller the hump. If the spacing were reduced to the ultimate--continuous support--the problem would be completely eliminated. Naturally, economics must be considered, and it would not be practical to space idlers so close as to approximate a continuous support. However, closer spacing to increase speed may have economic benefits.
The cross sectional load on the belt can be improved by adding more rolls to an idler, but additional rolls do not offer enough improvement to justify the added cost. A true catenary type idler with multiple rolls as supports could achieve the ultimate in cross sectional loading. Naturally, economics dictate the most practical number of rolls acceptable for long overland conveyor systems.